1. Field of the Invention
The present invention is related to systems and methods of operating a wireless network, in particular to a system and method select channels for communication that do not interfere with radar systems.
2. Description of the Background of the Invention
In a wireless network, for example, a wireless local area network (WLAN or WiFi LAN), the capacity of a particular channel is affected by signal strength, as measured by the Received Signal Strength Indication (RSSI) of the channel. Wireless communication devices operating in a wireless network may monitor the RSSI of a channel being used thereby for communication and if the RSSI of the channel drops below an acceptable level, such devices switch to another channel that offers a higher RSSI. The process of channel selection in this manner is called Automatic Channel Selection (ACS).
Wireless networks connect nodes via channels corresponding to certain frequency ranges. Standards using CSMA/CA technologies, such as IEEE 802.11n, define channels such that two neighboring networks may operate using two channels that correspond to frequency ranges that are adjacent (or close) to one another. However, in such cases the error rate of the transmission in each channel increases thereby reducing the channel capacity of such channels. It is therefore advantageous to be able to select channels (frequency ranges) with the least interference from neighboring networks as measured by a channel scan or a Clear Channel Assessment (CCA), which measures the ratio of time for which the channel is not in detectable use.
Typical ACS-capable devices undertake ACS at boot time, where the “best” channel is selected once before regular operation begins, and is not changed until the device is rebooted or the user deliberately triggers ACS. Some devices undertake ACS by randomly changing channels when the channel capacity of the channel being used thereby is detected to drop below a predetermined threshold. Such devices “wander” from channel to channel until a channel that has sufficient capacity is identified. Devices that undertake ACS in this manner may be appropriate for applications in which real-time transmission of data is not critical. However, such devices may not sufficiently support applications in which real-time transmission of data is expected. For example, a wireless transmitter and receiver that undertake ACS in the manner described above would not be effective for transmitting streamed video data from the transmitter to the receiver. If during transmission, the channel capacity used by such transmitter and receiver were to drop, the streamed data may be interrupted while the transmitter and receiver hunt for another channel that has higher capacity. Such an interruption may cause presentation of the video to a user to stall unexpectedly and result in a poor experience for the user. Some devices avoid interrupting transmission during ACS by having two radio frequency units in each device. In such devices, the two RF units operate simultaneously wherein one RF unit monitors the capacity of the channel being used to transmit data and the other RF unit monitors the capacity of the other channels.
Some wireless communication devices that operate in a wireless network that uses a 2.4 GHz frequency band estimate the capacity of a channel using scanning or CCA and perform ACS in accordance with such estimates.
However, wireless communication devices that operate in a wireless network that uses a 5 GHz frequency band may experience interference from or interfere with radar systems operating nearby in the same or similar frequency band. In some jurisdictions, devices that communicate using channels associated with certain frequency ranges may be legally required to detect radar systems that also operate in such frequency ranges and avoid using such channel if a radar system is detected. For example, the “ETSI EN 301 893 V1.5.1 (December 2008) Harmonized European Standard” for 5 GHz wireless networks specifies that devices that operate using frequencies in the ranges of 5.250 GHz-5.350 GHz and 5.470-5.725 GHz frequency ranges must detect and avoid radar interference. A wireless device may detect the presence of a radar system operating in particular frequency range by performing a channel availability-check (CAC), whereby the device switches to a channel associated with a particular frequency range and listens fix radar operation for a predetermined duration before transmitting any data. Typically, upon switching to a new channel, the wireless device may listen for radar operation for between 1 and 10 minutes before transmission. Alternately, the device may detect radar systems by performing an off-channel CAC, whereby the device listens for radar operation in a given frequency range without actually switching channels (i.e., while possibly transmitting over another channel). Once a channel has been selected, the wireless device continues to monitor the channel to detect newly activated radar systems with a certain probability.
Heidari et al., U.S. Patent Application Publication No. 2011/0096739, discloses a system that uses downtime (i.e., Off-Channel CAC) or multiple antennas to scan alternative channels and order the scanned channels in order of quality. The entire contents of Heidari et al. are incorporated herein by reference.
Kim, U.S. Patent Application Publication No. 2011/0150053, and Tsuruno, U.S. Pat. No. 7,715,801 are directed to low-level radar signal detection algorithms and the operation of a CAC module and, in particular, how a CAC may undertaken. The entire contents of Kim and Tsuruno are incorporated herein by reference.
Lin et al., U.S. Patent Application Publication No. 2010/0225530 discloses a method of interference-free radar detection in which a device send a clear-to-send signal to itself to reserve a period of time during which the device may perform radar detection. The entire contents of Lin et al. are incorporated herein by reference.
Matsuura, U.S. Patent Application No. 2010/0302966 discloses using one system for transmitting over a channel and another system for simultaneously monitoring the channel being used for transmission or different channel. The entire contents of Matsuura are incorporated herein by reference.
McFarland et al, U.S. Pat. No. 6,870,815 B2 (McFarland et al) discloses a scanning process for CAC that operates in the background. The scanning process uses beacons to clear traffic in the network for a period of time, increasing time between packets in the network, and reducing the network load. The entire contents of McFarland et al. are incorporated herein by reference.
Kurt et al., International Patent Application Serial No. PCT/IB2009/053746 is directed to developing channel selection data in a wireless network. The entire contents of Kurt et al. are incorporated herein by reference.
Birlik, et al., U.S. Provisional Patent Application 61/624,834 is directed to a system and method to equalize transmission delays in communication networks. The entire contents of Birlik et al. are incorporated herein by reference.
Birlik, U.S. patent application Ser. No. 13/460,139 is directed to a wireless network setup and configuration distribution system. The entire contents of this application are also incorporated herein by reference.